The problem of dew point corrosion in boilers
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Our company produces SO2 gas by burning sulfur (the gas does not contain any moisture). We are planning to have a 70 m2 waste heat boiler manufactured for us; the required inlet temperature for the boiler is 1050 degrees, while the outlet temperature should be 325 degrees (after calculations, the dew point for corrosion caused by SO2 in our company’s case is 270 degrees). The required steam pressure is 1.2 MPa. However, after contacting boiler manufacturers, they stated that the pressure in the boiler needs to be 2.5 MPa in order to prevent corrosion caused by the SO2 dew point, arguing that the steam pressure must be higher than the dew point temperature. In our company’s setup, the SO2 gas flows through the tube side of the boiler, with water flowing on the shell side. I have one question: if preventing SO2 dew point corrosion only requires the boiler’s outlet temperature to be above that temperature, then why is it necessary for the steam temperature to be even higher? Is it the manufacturer’s mistake or mine? Please explain this to me. Thank you!!!!
It’s easy to understand. Think of them as heat exchangers. If a heat exchanger is used to heat a certain medium with steam, can the temperature at the outlet of that medium be higher than or equal to the temperature of the steam exiting the exchanger? A temperature difference is necessary for heat transfer to occur. But in a flowing state, shouldn’t the temperatures eventually become equal?
According to thermodynamic theory, PV = nRT; here, nR is a constant. When the volume of steam remains constant (that is, V is also considered a constant), then P and T are in direct proportion to each other. Therefore, the requirements put forward by boiler manufacturers are reasonable
The flue gas outside the tube affects heat transfer inside the tube. Since the heat transfer coefficient of the tube for heat transfer inside it is higher, the temperature of the outer wall of the tube is closer to the temperature of the fluid inside the tube. If the temperature of the fluid inside the tube is too low, even if the temperature of the flue gas is above the dew point corrosion temperature, the temperature of the outer wall of the tube will still be below that dew point corrosion temperature, thereby causing dew point corrosion. I’m not sure if it’s clear.
What the original poster is referring to are fire-tube boilers used for producing sulfuric acid, where the flue gases flow inside the tubes while the boiler water is on the outside of them. On both sides of the tube wall of a fire-tube boiler, there is liquid-solid heat transfer (on the outer wall) and gas-solid heat transfer (on the inner wall). Obviously, the heat transfer coefficient of the outer wall is higher, and the temperature of the tube wall is closer to the temperature of the medium outside the tube, namely the boiler water. If the operating pressure inside the boiler drum is low, even if the temperature of the flue gas inside the tubes is high, the metal temperature of the tube walls may not remain far above the dew point corrosion temperature. Conversely, if higher operating parameters are adopted as recommended by the boiler manufacturer, and the boiler water temperature is already high, heating by the flue gas inside the tubes makes it easy to avoid the dew point corrosion temperature.